In the 1980s, many analog cellular communication networks were implemented over the world. These networks are already reaching their capacity limits in several service areas. This wireless communication technology has evolved from simple first-generation analog systems for business applications to second-generation digital systems with rich features and services for residential and business environments. There are several reasons for the transition from wireless analog to wireless digital technology: increasing traffic, which requires greater cell capacity; speech privacy; new services; and greater radio link robustness.
During the late 1980s and early 1990s, the rapid growth in mobile communications put a high demand on system capacity and the availability of the technology for low-cost implementation of cellular and personal communication services (PCS). CDMA has a larger system capacity than the existing analog systems. The increased system capacity is due to improved coding, gain/modulation density, voice activity, three-sector sectorization, and reuse of the same spectrum in every cell. CDMA is a cost-effective technology that requires fewer, less-expensive cells and no costly frequency reuse pattern. The power transmitted by the CDMA mobile stations averages about 6-7 mW, which is less than one tenth of the average power typically required by FM and TDMA telephones. Transmitting less power means longer battery life. CDMA can improve the quality-of-service by providing both robust operation in fading environments and transparent (soft) hand-off. CDMA takes advantage of multi-path fading to enhance communications and voice quality. In narrow-band systems, fading causes a substantial degradation of signal quality.
Since some new services, such as wide-band data and video, are much more spectrum-intensive than voice service, even the channel capacity improvement provided by CDMA will be depleted in the near future. This motivates some advanced wireless communication schemes, which can provide a greater capacity.
There are several problem areas with CDMA systems that are becoming more serious as the demand for greater capacity increases. Some of these problems are: spreading carriers; orthogonal functions; and synchronous considerations.
Spreading Carriers
In CDMA systems pseudo-random signals are used to: (1) spread the bandwidth of the modulated signal to the larger transmission bandwidth; and (2) distinguish among the different user signals which are using the same transmission bandwidth in the multiple-access scheme.
Ideally, these pseudo-random signals should be samples of a sequence of independent random variables, uniformly distributed on an available alphabet or range. In this case, the CDMA system is equivalent to a one-time pad used in cryptographic systems requiring the highest level of security. Since the key signal in a one-time pad should be as long as the message signal, it is not feasible to use it in CDMA.
A way must be found to store/generate good pseudo-random signals in both the transmitter and the receiver, despite the finite storage capacity/generating capacity of physical processing systems.
Orthogonal Functions
Orthogonal functions are used to improve the bandwidth efficiency of a spread spectrum system. In CDMA, each mobile station uses one of a set of orthogonal functions representing the set of symbols used for transmission. Usually, the Walsh and Hadamard sequences are used to generate these kind of orthogonal functions for CDMA.
In CDMA, there exist two different methods of modulating the orthogonal functions into the information stream of the CDMA signal. The orthogonal set of functions can be used as the spreading code or can be used to form modulation symbols that are orthogonal.
Synchronization Considerations
In a CDMA system, the heart of the receiver is its synchronization circuitry, and the heartbeats are the clock-pulses which control almost every step in forming the desired output. There exist three levels of synchronization in a CDMA system: (1) correlation interval synchronization; (2) spread-spectrum generator synchronization; and (3) carrier synchronization.
To correlate the Walsh codes at the receiver requires that the receiver be synchronized with the transmitter. In the forward direction, the base station can transmit a pilot signal to enable the receiver to recover synchronization. Just as the designers of the IS-665 wide-band CDMA system believed, with a wider bandwidth the base station can also recover the pilot signal sent by mobile stations.
What is needed is a system that will overcome each of these increasingly troublesome conditions. The (CD).sup.2 MA system of the present invention provides that solution.